They are just numbers. They could just as well have been other numbers.

This is how if/else statements work:

if (this expression is true) {
do this thing;
} else {
do this other thing;
}

The point of those numbers here is that 10<3 is always false which means that the else block of your if/else statement will always be processed. You could put anything that evaluates to false as your expression to get the same results.

In real life, you are not likely to use an expression like that. You would instead do something like age < 21 or password.length > 10.